Solid state disks, also known as solid state drives (SSD), and storage media such as USB memory sticks or flash cards, e.g., CF (compact flash) or SD (secure digital), are storage media capable of storing mass amounts of data. The drives, for example, can be used for storing data in computers or any other digital signal processing device similar to conventional hard disk drives. Unlike conventional hard disk drives, which employ a mechanically rotating disk having a magnetic coating, the solid state drives do not employ any mechanically moved parts. Instead solid state drives comprise integrated circuits for storing the data, wherein the integrated circuits comprise arrays of memory cells.
Several types of non-volatile memory cells are known. In one example, memory cells basically are made from double-gate transistors, which have two gates instead of one. A first gate is the control gate for controlling the transistor as known from conventional transistors. A floating gate is arranged between the control gate an the MOSFET channel area. The floating gate is surrounded by an insulating material. Any charge carriers, i.e., electrons, placed on the floating gate are thus trapped and will not be discharged under normal operating conditions. The memory cells accordingly maintain their status even if power is turned off, the memory thus being non-volatile.
Charge carriers, for example, can be placed on the floating gate by applying a high voltage to the control gate thus switching the transistor on and enabling a strong current from source to drain. The strong currents affect some electrons to jump onto the floating gate via a process called hot-electron injection. For removing the electrons a high voltage of opposite polarity is applied between control gate and drain for pulling off the electrons by so-called quantum-tunneling. This process of hot-electron injection and quantum tunneling, in particular, is used in NOR memory cells. Alternatively and, in particular, in NAND memory cells a process called tunnel injection can be used to inject electrons on the floating gate, i.e., for writing, and a process called tunnel release for removing the electrons from the floating gate, i.e., for erasing memory cells.
The memory cells can be connected, for example, in NOR or NAND architecture. In NOR architecture the memory cells are switched in parallel. Memory cells can be accessed individually. NAND memory cells are connected in series such that a plurality of memory cells shares one data line. The cells thus can be accessed in series only, such that for reading and writing the cells are accessed successively. Due to this architecture memory cells cannot be accessed individually. So for reading and programming, i.e., writing, all cells of a page are accessed.
In the here described example the transistor is the memory element storing the information. Note that memory cells comprising other memory elements can be used as well, for example memory cells comprising volumes of phase change material, i.e., PCRAM, or comprising other resistively switching materials, for example, magneto resistive RAM (MRAM).
In single state memory cells a cell is sensed to reflect one bit, i.e., the transistor is either conducting or non-conducting. In multi-state memory cells one cell may take more than two states. When the state of the memory cell is sensed, the amount of current passing through the transistor is sensed. A cell may accordingly take one of a plurality of resistivity levels and thus may reflect more than one bit, such that the cell is multi-level cell.
Unlike conventional storage devices comprising magnetic disks for storing data the above described flash memory in NAND architecture exhibits different characteristics regarding reading, programming and erasing data. Accordingly the principles developed for conventional storage media cannot be applied to this type of flash memory. Hence there is a need to develop fast and reliable flash memory devices.